Can an iPhone be used to digitize 35mm slides effectively?

Any decent camera with some degree of macro capabilities will be a feasible slide/negative scanner, but, tthere are some other factors that incide a lot in the results.

The first is an adequate backlighting device. Can be as complicated or as simple as you wish, as long as it allows you to get good exposure. I have tried different combinations of flash and lamps with paper diffusers. I happen to have a small lightbox (a lamp inside a box, with glass top and a difuser under the glass) and it has yielded best results.

The color temperature of the lamp will of course incide in the color balance of the obtained digital picture. The quality of the difuser is also important, if it has visible grain, it will appear as part of the scaned slide.

Some people recomends using a computer monitor. It can give you a lot of flexibility, but if you place the slide right over the screen, monitor's pixels will be visible trough the slide, so a suitable difusser must be added between the slide and screen.

Other people recommend placing the slide on a window, so daylight is your source. This can be handy for one time, emergency kind of work, but it can yield unrepeatable results as the light will be dependant on weather, time of day, and other objects outside reflecting color casts trhough the window.

Second is camera alignment. As you are takin a photograph of a square, you will notice the perspective distortion pretty easily. You must align lens axis perpendicularily and directly over the center of the slide to minimize distortion. Wowever, depending on your particular lens, you may get barrel/pincushion distortion. Post-procesing is the easiest way to correct in these cases.

For a moderate amount of scans, it may be advisable to build a relatively simple slide holder, so you don't have to fiddle every time trying to align the slide. It can be made out of cardboard or recycled boxes. But it can be made of more durable materials if you plan to do this a lot.

The holder can serve another purpose: Light shield. You mostly want all the light coming trough the slide, so no other light is contaminating your image. If you are backlighting a slide, light passing outside it can cause flare, and light inciding in front of the slide can reduce contrast and reveal too much of the possible surface defects on the slide. So the holder can be built to prevent both.

Finally, if you are using an iPhone or any phone camera, remember to clean the lens cover to get better results. Be patient and consider post production schemes, like good editting apps or transferring the files to a computer for color correction, perspective distortion correction, contrast and brigtness adjustments.

If your camera phone has a macro setting use it.